1. Field of the Invention
The invention relates to a component which can be subjected to high thermal loading and to which a hot-gas flow is admitted during operation, which component has at least one hot-gas side exposed to the hot-gas flow and a cold-gas side not exposed to the hot-gas flow, in which component passages are made, which passages connect the hot-gas side and the cold-gas side in such a way that a cooling medium flows from the cold-gas side to the hot-gas side, which cooling medium, when passing through a passage, absorbs heat from the component and draws it off to the hot-gas side, at least one of the cooling passages having an essentially circular cross section over its entire length, which cross section increases continuously from the cold-gas side to the hot-gas side, an opening half angle being enclosed, so that the at least one passage appears frustoconical.
2. Discussion of Background
In modern technology it is often the case that a medium flows around or over a component, the temperature of which medium far exceeds the maximum component temperature permitted in the interests of operational safety. Examples which may be mentioned are the conditions in the interior of gas-turbine combustion chambers and in their first turbine stages. Here, there is also a highly turbulent flow around the component, and this flow further intensifies the heat transfer. At the hot-gas temperatures achieved nowadays in the interest of high efficiency, the limits of the simple component cooling with a heat dissipation through the component are far exceeded.
Methods in which the direct contact of the components to be cooled with the hot-gas flow is prevented from the outset prove to be substantially more suitable. In the film cooling, which is common in gas-turbine construction, cooling medium—generally compressor air—is admitted to the component to be cooled on a side over which the hot gas does not flow, this side in the case of components around which flow occurs, such as blades, being the interior of said components. The cold-gas side and the hot-gas side are connected to one another by a multiplicity of passages, which as far as possible open out tangentially to the hot-gas flow on the hot-gas side. In this way, a layer of cool medium is applied to the component, this layer preventing the direct contact of component and hot-gas flow.
The so-called “showerhead cooling” represents a special form. This “showerhead cooling” is used in particular at locations where the hot-gas flow strikes the component to be cooled approximately perpendicularly. In this case, it is actually impossible to have the cooling passage open out tangentially to the main flow and thus form an effective cooling film. On the other hand, the cooling medium is directed at a comparatively steep angle to the component surface; the cooling medium absorbs heat from the highly loaded component while flowing through the cooling passage. The cooling medium is also successively brought into contact again with the component by the incident flow, so that this cooling method combines features of convective cooling with those of film cooling.
The cooling passages must be produced within relatively close tolerances for several reasons. On the one hand, compressor air is usually used as the cooling medium. The result of this is that, especially in the first turbine stages, which are subjected to extremely high thermal loading, only a small driving pressure gradient is available for the outflow of the cooling medium, especially at locations which are directly subjected to the free hot-gas flow, and at which stagnation points of the hot-gas flow form, thus in particular at the leading edge of the first blades of the first guide row. If there is not enough cooling air coming through the cooling passages, the cooling collapses. On the other hand, if too much cooling air is extracted from the cyclic process, this has adverse effects on the efficiency of the machine from the outset.
With regard to the throughflow, conventional cooling passages of constant cross section, in particular at low pressure ratios, are dependent on the pressure ratio to an extremely pronounced extent: the results of experimental tests show that the flow coefficient cd, which depicts the ratio of the mass flow actually fed through to a mass flow theoretically expected for a pressure ratio, has a pronounced positive gradient over the pressure ratio, in particular in the region of small pressure ratios.
This results in considerable difficulties at the leading blade edges, in particular of the first turbine guide row. Inevitably, operation must be carried out here with a very low pressure ratio. In addition, the hot-gas-side pressure may vary considerably due to slight displacements of the stagnation point of the incident flow. The consequence is the potential risk of a pronounced reduction in the cooling-air mass flow: this leads to component overheating in the region around the stagnation point, and thus to a pronounced reduction in the useful life of the component. Similarly low pressure ratios are also to be found, for example, at the cooling-air outflow at platform edges.
A common approach to the improvement of the cooling effect in film cooling is to configure the cooling passages as diffusers. Thus, U.S. Pat. No. 3,527,543 already specifies passages which expand continuously in the direction of flow of the cooling medium at an angle of 4° to 12° in order to reduce the cooling-medium velocity at the surface to be cooled and thus ensure a uniform distribution of the cooling medium over the component surface.
Furthermore, EP 0 648 918 B1 discloses cooling passages having a diffuser-shaped discharge cross section, the diffuser having a pair of surfaces opposite one another in the direction of flow, and the edge situated downstream curving away from the other in the direction of flow. In summary, the passage geometry specified there can be characterized such that a diffuser of rectangular cross section adjoins a passage section of round cross section, the diffuser of rectangular cross section opening out at the surface to be cooled. This achieves the effect that, inter alia, the undesirable velocity component of the discharging cooling medium normal to the component surface is reduced.
It remains to be stated that a multiplicity of cooling passages, the diameter of which ranges typically in the order of magnitude from a few tenths of a millimeter up to slightly more than 1 millimeter, are to be made in a component to be cooled, and that, especially if a defined diffuser effect is aimed at, a very high production accuracy is to be demanded. It furthermore remains to be stated that the diffuser geometry in the case of pure film cooling has considerable advantages, in which case the better wetting of the component surface with the cooling medium could be mentioned in the first place. It also remains to be stated that the better tangential application of the cooling medium, as outlined above, is unable to develop any significant effect in the immediate vicinity of the stagnation points of the hot-gas incident flow, but rather that the considerable task at these locations is to ensure that the cooling-medium mass flow, at a small pressure ratio, deviates as little as possible from the design mass flow.
This is because, in the case of the showerhead cooling outlined above, the reduction in the surface temperature by a cooling film which as far as possible comes into contact is not the primary aim, but rather the primary aim is to draw off the heat from the material as effectively as possible by means of the cooling medium flowing through the passages, for which reason purely cylindrical cooling passages are still the means selected in the stagnation-point regions of the components—thus, for example, at the leading blade edges or at platform edges. In order to ensure the design mass flow of cooling medium, the throughflow passages have so far been produced by a complicated process: the manufacturer is first of all given specifications for the cross section of the cooling passages which is to be produced. The component with the cooling passages made in it is then measured by virtue of the fact that the mass flow fed through at various pressure ratios is measured. If this mass flow deviates from the design mass flow, a new diameter of the passages is determined, and the component is reworked. This process may be repeated several times, until the mass flow of the cooling medium corresponds to the requisite mass flow. This results in a costly and time-consuming manufacturing process, one aim of the invention being to significantly shorten and simplify this manufacturing process and thus save, in particular, considerable costs.
Cooling holes whose throughflow behavior is less dependent on deviations from the design point also considerably increase the operational safety of a thermally highly loaded component and thus considerably increase the availability of a technical system whose essential part is this component.